The objective of the proposed research is to obtain information regarding two broad aspects of cellular differentiation: differential gene action and the formation of cell structure. The system being studied is the development of spermatozoa in Drosophila melanogaster. It is known that in these organisms certain aspects of sperm development are controlled by the genes of the Y chromosome. These genes appear to have no other function and are apparently activated at a very specific point in the life history of the male germ cell. In the absence of these genes defective sperm are formed in which certain ultrastructural elements are improperly organized or, in some cases, missing. Attention is being focused on the time and manner of activation of these genes, the nature of their products, the time of utilization of these products, and the role of these products in the formation of the ultrastructural complexity of the sperm-tail. Other male sterile mutants are being investigated in an attempt to discover other genes which are necessary for sperm development and to correlate the action of these genes with each other and with that of the Y-loci. Additionally the role and regulation of ribosome biogenesis during spermiogenesis is being studied in rDNA deficient males. The methods of investigation include electron microscopy, autoradiography, electrophoresis, density gradient centrifugation, and scintillation counting.